Fiber-optic cables are used in a wide variety of applications today to replace traditional copper cables. Such fiber-optic cables, for example, may be utilized to transmit data and control signals between computers and processors. Optical fiber provides reliable data transfer, with exceptional speed and bandwidth. The small size and the light weight of fiber-optic cables make them particularly useful in communication applications, which have significant space and weight restrictions.
Fiber-optic cables receive significant use in the aerospace industry for both commercial and military applications. In such usage, the fiber-optic cables must have a very robust construction because even minor failures in a cable may have significant and undesirable consequences. Generally, in traditional aerospace usage, the construction of a fiber-optic cable includes a glass strand, or fiber, and a cladding layer and buffer layer. One or more buffer layers are utilized on the outside of the glass strand for physically supporting and buffering the fragile glass strand. Furthermore, since fiber-optic cables are often subjected to extremes in temperature, pressure, vibration and shock, additional layers, such as strength member layers and jackets or jacket layers, are utilized on the outside of the buffer.
Fiber-optic cables also might utilize fibers made of plastic or polymer, referred to as plastic/polymer optical fibers (POF). Generally, such POF fibers are made of polymethyl methacrylate (PMMA) for the optical core. The POF fiber then might be coated with a thin coating of a material, such as a fluorinated polymer, as a cladding layer. The POF cable may also have other buffer layers over the fiber and cladding layer. Generally, such traditional POF fibers are not suitable for use in aerospace cables with their current constructions. Aerospace cables are subjected to significant mechanical, fluid, environmental, thermal, and other stresses, and so require a robust construction. Furthermore the FAA requires that wire and cables utilized in aerospace applications pass a flammability test referred to as the FAA Flammability Test per Appendix F, part I of 14 CFR part 25 for wire and cable. Therefore, while typical POF fiber cables have been found suitable for automotive, electronic, and household uses, the current products will not meet tougher aerospace requirements. Furthermore, even if the POF fiber elements are constructed with typical aerospace components, because of the materials and construction of the actual POF fiber, they still will not pass the noted flammability test.
Fiber-optic cables made using POF fibers as the core are flammable since PMMA is a highly-flammable material. Even enhanced POF fiber cable construction, using outer layers that have some inherent flame resistance and with typical strength members like aramid and glass yarn materials surrounding the fiber elements, does not solve some of the issues presented by POF cables, due to the flammability of the core material. The resulting cables still fails a FAA flammability test because the strength members burn away, exposing the highly-flammable inner components that will continue to burn and/or allow flaming droplets to drip out of the construction that is not protected by the strength member.
Fiber-optic cables used for aerospace applications are typically manufactured with materials in each layer that help protect the cable. Along with the flammability requirement, the fiber-optic cable is exposed to temperature extremes of +100 to −55 degrees C. or more. Furthermore, such cables are exposed to hydraulic fluids, jet fuels, cleaning solvents, runway deicers and other corrosive fluids.
Also, in fiber cables that incorporate both glass and POF constructions/core elements, it is desirable to use layer materials that are flammable due to the greater flexibility of the material, the lower cost, and ease of the manufacturing processes. To try and address some of the issues noted herein with different buffer layer materials that have some flame resistance makes the cables more costly, less flexible and with a greater tendency to shrink.
Furthermore, the materials used that can address the harsh exposure of such aerospace cables still are not assured to pass the FAA flammability test. As noted, Plastic Optical Fiber (POF) uses polymethyl methacrylate, known as PMMA, for the optical core. The high flammability of the material prevents it from passing the FAA flammability test when tested alone, and even when it is surrounded or jacketed with inherently flame-resistant materials. Even adding strength member layers as commonly used in aerospace cables, such as para-aramid yarns and common fiberglass (e.g., E-glass), does not allow the finished product to pass the FAA flammability test.
It is therefore an objective of the invention to improve generally upon existing fiber-optic cable technology and to provide a fiber-optic cable with a robust construction for aerospace uses. It is further an objective to provide a POF cable that is able to be used in aerospace applications. It is still further an objective to provide a cable that is able to withstand flammability requirements, including the FAA flammability test.
These features and other features of the invention will be come more readily apparent from the Detailed Description and drawings of the application.